In the art of electronics, needs for high density packaging are increasing greatly, and accordingly in those technical areas in which flexible printed circuit boards (hereinafter referred to as FPC ) are employed, need for high density packaging are becoming greater as well. In the process of producing FPC, film dimensions are liable to appreciable changes at the stage of etching, at which the copper foil stuck on the base film is etching as the required pattern, and/or at an adjacent stage. Therefore, it is required that dimensional changes before and after such stage are minimized in order to enable high density packaging.
Hitherto, polyimide films have been used as a base film for FPC. Of these films, polyimide films having good handling property and flexibility have a smaller modulus of elasticity and a greater linear expansion coefficient. So, these polyimide films have not been satisfactory enough to meet the needs for high density packaging. A polyimide having a smaller linear expansion coefficient and a greater modulus of elasticity can be synthesized, but such polyimide film is much less flexible and can hardly be used as FPC. A polyimide film which have good dimensional stability with low thermal shrinkage and low linear expansion coefficient is produced by film drawing. It is disclosed in Japanese Patent Application Laid-Open Publication No.61-296034, but this polyimide film has not yet had mechanical properties, such as sufficient elongation.
FPC fabrication is carried out on a roll-to-roll basis such that FPC base film and copper foil, both are wound on respective rolls, are paid out respectively and heated for being laminated together. Because of this, both base film and copper foil are subjected to tension in the direction of being fed from the respective rolls, that is, in the mechanical feeding direction( hereinafter referred to as MD direction), and under such conditions FPC has been produced.
Then we'll consider the base film. In MD direction it is subjected to both elongation under tension mentioned above and elongation due to thermal expansion under the heat applied during the stage of lamination. On the other hand, there occurs a phenomenon that the film shrinks in a direction perpendicular to the Mechanical feeding direction by an amount corresponding to the amount of its elongation in MD direction due to the tension. Accordingly, in TD (transverse direction) direction shrinkage is given to the film.
The copper foil is little subjected to deformation under the tension applied in the usual process of FPC making, because the copper foil has a very large modulus of elasticity. It is only through thermal expansion under heat applied during the stage of lamination that the copper foil is deformed. Thus, elongation due to thermal expansion in both MD and TD directions is given to the copper foil.
Therefore, if the sum of elongation of the film due to the tension and elongation of the film due to thermal expansion in MD direction is equal to the elongation of the copper foil due to thermal expansion, the distortion are set off against each other. Likewise, in the TD direction if the sum of shrinkage of the film in TD direction due to film's elongation under tension in MD direction and elongation of the film in TD direction due to thermal expansion is equal to the elongation of the copper foil due to thermal expansion, the distortion are set off against each other.
However, since the base film and the copper foil are isotropically produced, an FPC, where both base film and copper foil are laminated, remains considerably distorted in TD direction when the base film is secured to the copper film. Therefore, when the copper foil is removed by etching for pattern formation, the strain which has been locked is released and accordingly there will occur considerable dimensional changes before and after the stage of etching, which can seriously hinder high density packaging.
Therefore, the present inventors directed their research efforts toward developing a resin film, particularly a polyimide film which is little liable to dimensional changes before and after the stage of etching in the process of FPC fabricating, and a method of making same. As a result, this invention was arrived at.